text:
What does "1000ft above" mean in the context of VFR flight minimums?
There are a bunch of VFR flight minimums. My question here is the follows:
Lets say one of the minimums is '1000 feet above'. Does that mean.
A. The flight should be 1000 feet above clouds?
B. The clouds should be 1000 feet above the flight?
Different websites on google are interpreting it differently.
Until today I thought it is 'A' .... but recently came across a FAA knowledge test question on Sportys, where the answer is follows.

No person may operate an aircraft VFR within the lateral boundaries of
Class D airspace with a ceiling less than 1,000' and visibilities less
than 3 miles.

Here it says ceiling less than 1000'. Which by interpretation, one should not fly a flight under VFR (class D), with less than 1000' below clouds. This says the right interpretation is 'B'.

Can you quote where you are getting the info from? It sounds like you are conflating two different regulations. Cloud clearance and ceiling are different things.

Ohh...I did not know cloud clearance and ceiling are different things. Please link me to the two different regulations you are talking about.

Here is the clearance requirements. There’s a pretty good diagram here. The regs are under 14 CFR 91.155.

I think you're getting the concept of a "ceiling" confused with cloud clearance regulations. They're two different things.
A "ceiling" is a layer of clouds that is greater than a certain density. At least in Class D airspace, you can't fly VFR at all unless either there is no ceiling, or the ceiling is more than 1,000 feet above the ground.
If it is legal to fly VFR, then you still have to stay a certain minimum distance from any clouds, regardless of if any given cloud is part of a layer or not. So, a single cloud in an otherwise clear sky wouldn't constitute a layer, much less a ceiling, but you do still have to stay away from it.
The phrase "1000 feet above" means you are the one that's above the cloud, not the other way around.

Can you provide a URL to the ceiling requirements

@JeshKundem 14 CFR § 91.155 (scroll down to part c). Also, the definition of "ceiling" is in 14 CFR § 1.1.

Re -- "You can't fly VFR at all unless either there is no ceiling, or the ceiling is more than 1,000 feet above the ground."-- generally speaking, this is not true.  You are referring to FAR 91.155c which only applies " within the lateral boundaries of controlled airspace designated to the surface for an airport".  I.e., in areas where Class B, C, D, or E airspace comes all the way down to the surface.

(And, arguably, excluding surface-level Class E extension airspace -- see https://aviation.stackexchange.com/q/74738/34686 .)

(Granted, the question was specifically about Class D airspace, but you meant the answer to also be restricted to this also, it would help to say so-- e.g. "In Class D airspace, you can't fly VFR at all (unless you receive authorization for SVFR) unless there is either no ceiling, or the ceiling is more than 1,000 feet above the ground.")

@quietflyer Good point. Done.

Consider how the “500 feet below [clouds]” requirement of 91.155(a) interacts with the “500 feet above ground” requirement of 91.119(c). The logical result is what 91.155(c) says, but saying it explicitly also bars the “except as necessary for takeoff and landing” in 91.119 from being used as a loophole to do stupid, dangerous things.
If the ceiling is 1000ft AGL, then it would be technically legal* to fly VFR at exactly 500ft AGL. However, if the ceiling were even one foot lower, then there is no way to legally fly VFR.
(* Note that “legal” does not mean “safe”. Do not try this at home.)


text:
Is Flight Service Station the same as En Route Flight Advisory Service?
As title suggests, is "Flight Service Station" the same as "En Route Flight Advisory Service?"
In regards to VFR, I was told that if I get lost in flight and general lost procedures for VFR don't work then I can resort 121.5 or contact flight service station. When I did some digging, I also found that the other option to contacting 121.5 is 122.0. From there, I found that 122.0 is EFAS. Hence, is EFAS the same as Flight Service Station?
What exactly would the phraseology sound like? "Something-something  heading towards X. I'm currently lost and unable to triangulate. Please advise?"

To start, no, "En Route Flight Advisory Service" and "Flight Service Station" are not the same thing. They can never be the same thing because they are entirely separate categories of thing—you are comparing apples and oranges, or more like apples and apple farmers. EFAS is a service and an FSS is the entity which provides that service; it is like asking if "keeping the roads clean" and "the Department of Public Works" are the same thing.
Next, En Route Flight Advisory Service, commonly known as "Flight Watch," does not exist anymore. This service was primarily used to provide pilots with in-flight updates on hazardous weather along their route; with the advancement of weather radar (air-, ground-, and space-based) and better in-aircraft display options it was decided to terminate that service in 2014 (see also this NBAA article which incorrectly defines EFAS as Emergency Flight Advisory Service). "EFAS" is listed as a term in JO 7240.2E (effective July 2014) but is not defined in JO 7340.2F (effective October 2015).

121.5 (VHF) and 243.0 (UHF) are the "emergency" or "guard" frequencies. They are monitored by ATC facilities across the country, and possibly by FSS as well. In addition, all pilots are required by FDC NOTAM 4/4386 (PilotWeb, PDF) to maintain a "listening watch" on these frequencies, if capable. You should use Guard to establish communications in the absence of any other clearly available communications channel, for example if you are a VFR pilot not receiving ATC services and you have an emergency, or if the ATC frequency you had been using becomes blocked by a stuck microphone.
Flight Service frequencies are described at many places around the Internet, for example this AOPA article (which is from 2008 and references EFAS and TWEBS which don't exist anymore).

I'm not entirely certain how Flight Service would be able to help you determine your location. It is technologically possible to triangulate the source of a radio broadcast, and some ATC radar displays in Europe apparently do this to highlight the data block of the aircraft currently transmitting—but ATC in the United States does not have this capability, and if ATC doesn't have it I imagine Flight Service doesn't either. Indeed, it's usually the opposite: because Flight Service frequencies are duplicated across the country (in order to provide constant coverage) you must tell them which of their receivers you are trying to engage with and which of their transmitters you are listening on! Not the most helpful thing if you don't know where you are yourself.
If you are truly lost—you have no iPad with a GPS moving map on it, you are above an overcast layer and cannot use pilotage, and you failed to properly pre-plan your flight so you have no idea of which ATC frequency is available in your location—you can call ATC on Guard, something like this:

You: Center, Skyhawk 12345, on Guard, I am lost and require location assistance.
ATC: Skyhawk 12345, Podunk Approach, ident. [you do]
ATC: Skyhawk 345, radar contact three-eight miles north-northwest of Podunk VOR. Change to my frequency 121.9.

Assuming you have a transponder...

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text:
After an aircraft releases the parking brakes, can the pushback vehicle's brakes hold it in place?
When pushback is connected to the nose gear, can the pilot release the aircraft's parking brakes while the pushback vehicle's "hand-brake" is set? Can it hold the weight of the aircraft?
Any regulations or SOPs that are covering this question?

On flat ground? On a hill? Sitting on ice? Engines idling? Engine run up?

@Ron Beyer, At gate while engines are off (flat ground).

Then yeah, gravity and static friction will do most of the work there... If you put your car in neutral on a flat driveway, does it roll away? Even small chocks will stop any movement.

On flat ground, you can release the brake & the aircraft won't roll anyway. With enough slope, it could roll. (But the tug, brakes set, would prevent that.)

Thanks for your replies.. The last question please; Do pilots & ground staff do it, say if there's a delay or traffic behind and the pilot has been asked to release the parking brake, the tug set its own brake or they ask the pilot again to set the parking brakes? Is there any SOPs or regulation to prevent doing that?

Your 2nd question in the comments deserves to be its own question. That's how the system here works - one question per "question" box. Questions/answers in the comments can get lost in the shuffle or even deleted. You'll learn all this and more if you'll spend a minute or two taking the [tour].

What particular aircraft and vehicle? Not really answerable because it depends on many factors.

The tug operator talks to the pilot through an intercom connected directly into the airplane's audio system, and when the pushback is ready to start, the tug will confirm PB release with the pilot before the movement starts.
If the tug operator doesn't have voice communication with the crew, hand signals are used, parking brakes off being two closed fists held up, then hands opened with palms toward the pilot (brakes on is the opposite).
The tug is quite heavy, with powerful brakes, and has no problem slowing down or holding the mass of the aircraft at the walking speeds being used.
If the tug driver has to stop during the pushback, the driver will carefully slow and stop the tug, and depending on how long they expect to wait, or if some other conditions exist like a slope or a strong wind that wants to push the plane along, may tell the pilot to reset the parking brake as well.  Or they may just sit there relying on the tug's brakes, especially if it's expected to be a short 10 or 15 second wait. Depends on the situation and the operator's judgement, and airline SOPs that dictate minimum actions.
SOPs for that will depend on the airline.  You also have manufacturer's towing (and taxiing) procedures, covered in Chapter 9 of the Aircraft Maintenance Manual, and airline SOPs will typically be based on that, possibly with their own custom procedures thrown in.

Yes, they can. Planes generally* don't have any kind of automated communication with the tug. The brakes, power, etc. are all coordinated by hardwired (voice-only) intercom, (voice-only) radio, hand signals, signal lights, or something similar. So there's no interlock that prevents the pilot from doing whatever they want with the brakes at any time.
Airplane tugs are big, heavy things with oversize brakes, specifically because they're designed to move bigger, heavier things. Typically, the airplane's brakes aren't used at all during the pushback, the tug does all the braking itself. So, yes, its brakes would be quite capable of keeping the plane still without any help.
As far as I'm aware, there are no specific regulations on the use of plane brakes vs. tug brakes. Rather, the airplane's manufacturer and/or the airline will have a set procedure to use, and there may be (depending on jurisdiction) a regulation that says you must follow procedure.
*I don't actually know of any exceptions to this, but I'm not exactly a guru of all things pushback-related either.

You've never seen a ground person hooked up to the aircraft with a headset? It's pretty common... https://aviation.stackexchange.com/questions/13816/how-are-pushbacks-coordinated-between-the-pilot-and-the-tug

@Ron, I don't see anything in Hidden's answer saying that wasn't an option. They said that there generally isn't automated communication between the tug and the plane, e.g. an interlock, and that instead everything is coordinated by radio or hand/light signals.

What is meant by "automated communication"?

@MichaelHall Maybe I phrased that wrong. I was referring to some kind of interlock that would let the tug (or the tug driver) control the plane's brakes directly, as opposed to talking to the pilot and having them set or release the brakes.

@RonBeyer I was including that under the "something similar". I'll add that to my answer so as to be more clear.

Well, I don't think "automated communication" is a useful term here.  It doesn't add any clarity, and the meaning is fuzzy in almost any context.  Communication, (by definition and between humans...) really must be a manual process to transfer thoughts to another person.  (via speech or text, aided by intercom, radio, etc.)  What is there to "automate"?

@MichaelHall In a big truck, when you step on the brake pedal, it applies the trailer's brakes as well as the cab's. This happens automatically, without the driver needing to select it. Yes, it's done by air pressure rather than electronically, but the principle is the same. It would be pretty simple to do something similar in aviation, with a tug automatically applying the plane's brakes via a data link when the tug's driver presses the brake pedal. The OP asked if the pilot could control the brakes independently of the tug. The answer to that is "yes", because such a link doesn't exist.

@HiddenWindshield: OK, I get your point now.  I just wouldn't call electric trailer brakes "automated communication". (I interpreted "communication" to mean human coms, like plugging in a headset...) Maybe there is a better way to state this, but it's up to you if you want to edit more. Just my two cents worth.

@MichaelHall Yeah, that might not have been the best word to use, but I was trying to be inclusive of such things as a computer on the tug communicating with a computer on the plane to control the brakes. Like I said, if you've got a better way to put it, I'd be happy to edit.

@HiddenWindshield, maybe something like: "there's no interface or controller that allows the operator to apply the aircraft brakes from the tow vehicle"?

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text:
What protects the location beacon (pinger) of a flight data recorder?
The pictures of flight data recorders show that the location beacon (pinger) is outside the main protection encasement:

Is it really a good idea to keep such a vital instrument unprotected? Or is that round metallic thing strong enough to resist impact and fire?

it doesn't need to survive a fire (it'll sink) only impact with water

Also, the more stuff you cover it with, the harder it may be for the "ping" to get out.

Is it really a good idea to keep such a vital instrument unprotected? Or is that round metallic thing strong enough to resist impact and fire?

The casing of the ULB is itself designed to be strong enough to withstand the forces expected when an airliner crashes into an ocean, lake or large river (which are the only circumstances where a ULB is useful).
See teardown video starting at around 4:45
Copies of the relevant standards cost $70 each. However, a 1968 report from the early days of ULB development says

As voretaq7 notes in a comment: ULBs need to be external to the FDR/CVR canister because they need to have batteries changed regularly without affecting the integrity of the data recorder canister.

The "pinger" also needs to be replaceable/serviceable - they have a battery with a limited shelf life which is much shorter than the service life of the recorder. Were the ULB located inside the main body of the recorder the entire assembly would need to be returned to the manufacturer to be opened and services - this design allows airline maintenance to replace the ULB without compromising the seal on the data portion of the FDR.

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text:
Why are videos call and voice calls prohibited on airplanes?
I'm taking a flight to Japan this summer and noticed that on United, they may have wifi but it says everywhere that no voice calls or video calls are permitted and strictly prohibited during flight. They don't say why, but out of curiosity why is that such a big deal?

To be clear, are you asking specifically about calls over WiFi?

My feeling is that this questoin belongs to travel.SE (related to traveler issues). Aside from possible the technical reason (bandwidth limitations and latency due to satellites links), there can be commercial reasons (they may sell voice call services and want you to use them)

I'm voting to close this question as off-topic because it belongs on Travel.se.

Voice/video calls have not been approved by the FAA, and no one is particularly interested in having them approved, due to the frustration it would cause amongst passengers, having to listen to a multitude of conversations in a confined space, throughout the flight. Not to mention the fact that it would clog the limited bandwidth if even a sizable chunk of the passengers wanted to do it.
Virgin comes right out and says it with something along the lines of "For the safety and comfort of our guests, VoIP and Video Chat is also prohibited".

The FCC does not approve of cell use and airplanes. One of the reasons is it wreaks havoc on cell systems because of frequent cell changes.

The question is asking about Skype-style calls over the in-plane WiFi network, not about cellphone calls.

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text:
How to request to fly on a flight level, different from the direction, according to the charts?
According to the direction, there are correct FL to fly, but due to weather, or traffic, the pilot would request a different FL, from the chart (according to the direction). Is there any correct phraseology to request it?

If you are assigned an altitude to fly then you are being separated from other traffic. If you are not assigned an altitude you are not under ATC control and altitude is at your discretion, you would simply inform ATC of your level change. I'm not sure I understand what you are asking.

"XYZ Center, 123-Alpha-Beta With Request"...

@RonBeyer please excuse my lack of knowledge but in Aviation world isn't it Bravo instead of Beta?

Say you are flying on a route that requires an even level, but you would like an odd level such as FL350. The phraseology to use would be

Request FL350

The rest is up to ATC. Depending on local agreements and the current situation, we may be able to grant you the level right away, maybe we need to pick up the phone and talk to downstream sectors or maybe we can't offer the level at all. In any case, there is no harm in asking.
Regarding the part of your question that mentions weather, if you are requesting a level change because of extreme weather (such as moderate/severe turbulence or icing), state so in your request. We will then handle your request with higher priority, and you are more likely to get the different level.

Request FL350 due to moderate icing

I often hear pilots say, "request wrong way FL350" as a way to convey, I know that I'm asking for the opposite direction, but that is what I actually want to request.  It isn't in the book (AFAIK), but it's pretty common.  And, more often, the transmission is more along the lines of, "We're right in the tops here at FL340, and we're too heavy for FL 360, but would wrong-way FL350 be available?"

@RalphJ good point.  Been there and done that.  Usually I don't say "wrong way", usually I just say something to convey the operational consideration.  Another approach is a short form PIREP, "moderate icing at FL280, requesting FL310."

@HankyPanky Because OP specifically asked about requesting a non standard level

@HankyPanky if Ralph J's comment isn't sufficient to answer your question, feel free to ask a whole new one!


text:
What is the correct use of the verb "to leave," for exiting the runway?
What's the correct way to say, according to the phraseology;
I will leave the runway; I will vacate the runway; or I will exit the runway?

What jurisdiction?  In FAA and ICAO land, it's "vacate".

What region are you asking about? The FAA Pilot-Controller Glossary doesn't specifically define either term, but it does use phrase "exit the runway" several times while never using the phrase "vacate the runway" (though it does use 'vacate' in regards to an altitude in one place.) On the radio, though, I usually hear 'vacate.'

If is is possible, from FAA and ICAO, please. I am from Brazil.

The correct term is to vacate a runway. The term is used several times in the ICAO Manual of Radiotelephony (Doc 9432), including the following example (among many):

4.9 After landing
  Unless absolutely necessary, controllers should not give taxi instructions to pilots until the landing roll is completed. Unless otherwise advised, pilots should remain on tower frequency until the runway is vacated.
Pilot: Georgetown Ground Fastair 345 runway vacated
  ATC: Fastair 345 taxi to stand 27 via taxiway alpha

And a relevant note from the same document:

Note.— The runway is vacated when the entire aircraft is beyond the relevant runway-holding position.

More often than not though I will hear a controller say "please vacate the runway as soon as possible", and then the pilot will announce "N12345 Clear of Runway XY", I don't think I've ever heard a pilot say that they've "vacated" a runway. In fact "Clear of the runway" is in the pilot/controller glossary as such.

@RonBeyer Where? In the USA? The FAA has a tendency to deviate quite significantly from international standards (ICAO). "Vacate" is definitely used in countries following ICAO standards - cf. Doc 9432 as well as my personal experience

Yes, in the US.

Vacate is definitely used... and as for "Unless absolutely necessary, controllers should not give taxi instructions to pilots until the landing roll is completed", well - https://youtu.be/x23fBpxuWGM

In our uncontrolled airport we just radio to local traffic that we are clear of the active runway.

I've never heard the term 'vacate' for this, though there's no reason they cannot do so.  Most controllers will say 'Exit runway when able' or at a specific intersection 'Exit at Alpha two', etc.  as a pilot, I have always said that I am 'clear of the active' followed by my taxi request.


text:
What is the purpose of a Victor Airway?
I see lots of great questions related to Victor Airways, but I haven't seen any which indicate why someone would want to fly one or what really makes them special or unique.
I get that they are radial of a VOR to another VOR and that certainly provides an indication in determining what that radial is if you wanted to fly from one VOR to another, but I do not understand the value of defining it as an airway beyond that.
What is the purpose of indicating things such as width of the Victor Airway, or altitude or any other attribute? Is the concept that it would be frequently travelled and thus these attributes make it a little more controlled and thus safe?
In short: Other than providing an easy way to determine a radial from one VOR to another, what is the purpose of a Victor Airway?

When doing IFR or cross-country VFR, you can use Victor Airways as a "highway" system. It makes it easy to describe your path in a flight plan as well, especially if you need to avoid certain airspace.

Related

Is it safe to say that these airways are meant for IFR traffic to fly? As a VFR pilot, I have never flown an airway.

An airway (Victor or otherwise) is just a standard route for aircraft to fly on in the national airspace system - for all practical purposes they are literally highways in the sky (with the lanes being stacked vertically rather than horizontally).
This is part of the paradoxical logic of how ATC provides separation services for IFR aircraft: "You have to put them together to keep them apart." -- In an environment with no radar, or in crowded airspace, having everyone following a standard routing ensures that you know what each plane is going to do. If you have 10 aircraft all flying along an airway and each separated by altitude or time you know they won't hit each other
We define the attributes (width, minimum altitude, etc.) of a victor airway mainly for purposes of obstacle clearance:
A VOR has a specific degree of precision in its measurement, and if we assume the maximum distance from the VOR (the widest degree of arc) and the maximum allowable "slop" in the VOR receiver/indicator in the aircraft that means an aircraft that thinks it's "on the airway" will be somewhere within a given width around the defining radial: For Victor airways that width is 8 nautical miles - 4NM on either side of the centerline radial - or 4.5 degrees if the VORs are more than 102 nautical miles apart.
Those numbers may sound  like they're arbitrary, but they aren't: If the VORs are exactly 102 nautical miles apart then the width of the airway at the 51 nautical mile midpoint between the two VORs is - you guessed it - 4 nautical miles. The further apart the VORs are the wider the center of the corridor becomes, accounting for the fact that a degree of arc gets wider as you get further from the VOR.
Airways are an IFR construct so if we continue on the assumption that the plane is in the clouds and the pilot can't see anything then within the airway corridor the aircraft needs to be clear of any obstacles (towers, trees, etc.), and of course it needs to be able to receive the VORs that define the airway. The combination of those factors and a few others will determine the minimum altitude at which a given airway can be used.

Note that much like ground highways airways are not always the most direct or convenient way to fly from one location to another. In particular the Victor airways are defined by VOR radials (or the intersections of such radials), but they're not always a straight line between two VORs.
As an example, take a look at this utterly contrived flight plan for the purposes of illustration: The airway I picked (V451) will take me from Groton VOR (GON) to La Guardia VOR (LGA), but it bends in three places on this route: CREAM, KEYED, and NESSI (not labeled on the sectional, but its the bend southeast of Bridgeport). A straight line between the two VORs would be the 265 radial from Groton direct to La Guardia: A much shorter distance, but not a "standard" airway routing.

Separating planes on the same airway is trivial. What is the main advantage is that you can have a prepared index of which airway intersects which others. That allows quickly and easily checking which flight-plan conflicts—and therefore can't use the same flight level—with which others even when they have completely different routing.

To add to @voretaq7's answer, there are a few more benefits of airways over direct VOR to VOR routing:

Airways can bend a locations besides a VOR (e.g. V248 between SNS and PRB VORs bends about 10 miles from PRB).
Airways can span across several VORs, much like the interstate system. For instance, V27 starts at Seattle, and follows the west coast all the way down to San Diego, passing through several VORs, and several turns along the way. If someone's flying IFR, the flight route could simply be "SEA V27 SAN" and everyone concerned knows exactly what it means.
Airways also serve to provide obstacle clearance, and often it's quite a bit lower than the OROCA in the neighbouring (usually, mountainous) terrain. If you're flying off airway, you need to fly much higher, which may be outside your aircraft's capabilities


text:
Why are there so many 7700 squawks?
I installed an app some months ago, which gives a push message when an aircraft squawks 7700. The first time it happened, I thought the plane was about to crash, but fortunately nothing happened.
However, within the last months, I got a lot of notifications, maybe a few a week or so. I'm wondering, why there are so many emergency alerts?
I've read about possible reasons for 7700 squawks and they seem to be manifold. Isn't the purpose of an emergency call to be used only in really critical scenarios? How is an aircraft going down to be differentiated from one carrying a sick passenger? Are there any plans in the industry to change the current system?

What app is that?

Sorry, I can't remember anymore!

"Are you referring to "lots of notifications" (7700 squawks) in the entire U.S. or a selected area with the U.S.?" -- or worldwide, or what?

7700 is a "general emergency" squawk. It tells ATC that there is "a problem" of some sort with a particular plane. And I agree that this "general umbrella" type of squawk is a good idea. I have several disagreements with the FAA, but this is a good one. Several thoughts:

the specific nature of the emergency is only for the PIC and ATC. Nobody else needs to be informed, they can't help anyway.
oftentimes lost comm is part of the problem. So they can't talk, but you still want to show ATC that you have an emergency. You could squawk 7600 for lost comm, but if you have some truly serious, like an engine failure (I personally don't consider lost comm anything even remotely near "truly serious"), you can inform ATC with 7700 that you have a more serious problem than just lost comm. They'll notice the "lost comm" aspect of this situation on their own, by not being able to talk to you. Now they know you have something more serious, because you don't show 7600.
If you really want to use different squawk codes for different types of emergencies, it would merely add to the already large amount of numbers and codes pilots and controllers have to learn by heart. In an emergency where time might be of the essence, trying to remember abstract information like this can be difficult.

"The first time it happened, I thought the plane was about to crash"
Why did you think that? 7700 is not some "we're gonna die" type of squawk. It's a general "umbrella type" of squawk for all sorts of emergencies. An intelligent pilot will squawk 7700 for any "small" or "big" type of emergency (and who is supposed to be the judge on "big" vs. "small"?).
"How is an aircraft going down to be differentiated from one carrying a sick passenger?"
It isn't, because it doesn't matter. The people that are familiar with the situation (PIC and ATC) are in communication, unless lost comm, in which case the controller who can't glean more info from the 7700 squawk can't help anyway, and nobody else needs to be informed because they can't help anyway. Squawk codes aren't meant for desktop people like you who download some app and watch.
"Would you recommend handling this issue differently than it is done now, if you were in the position to?"
No. There are good reasons to have an "umbrella" type of squawk for all types of emergencies.

More than just ATC can help. If you squawk 7700 and talk to no one you can still expect to see a fire truck and ambulance where you land. Might not help in the air but you will appreciate it if you need it.

True, but that doesn't matter for squawking. You don't squawk for fire trucks. Squawking is for ATC.

True but priority handling, getting other aircraft out of your way and coordinating with ground emergency services is all part of the deal that ATC provides.

True, but that's not part of the squawking policy. The question is about the regulatory situation, not additional benign, auspicious side effects. We can appreciate positive side effects, it doesn't mean they're the reason for the regulation. The question was about "the purpose of ...". That's about ATC policy, not benign side effects such as fire trucks.

Meta: While the arguments against specific emergency squawks are valid, this answer hasn't acknowledged a single positive aspect of them (read: there are two sides to a medallion), hence I feel this answer is biased.

@mucaho but the question has nothing to do with specific squawks. Why would this answer be biased against them?

@egid I stand corrected, better choice of wording would have been "biased against the issue being handled differently than it is now". In hindsight this may have been a bit to harsh / without basis, guess I have understood it like "no, this is how it is, everything else is bad".

"The first time it happened, I thought the plane was about to crash" – If the pilots still have time to fiddle about with transponder codes, they are very probably not about to crash.

Yepp, precisely!

Outstanding answer, but a few of your comments seem to indicate an odd perspective:  “Squawking is for ATC”.  True from the standpoint that only ATC sees it, but it is intended to benefit the crew.  Better might be “it is for getting the attention of ATC”.  And if the crew requests fire trucks that would not be a “side effect”, it would be the primary response to a declared emergency!  (Especially considering also lost comm, as was mentioned…)

The reason for squawking 7700 is that Air Traffic Control can easily identify you on the radar display. This eases identification and coordination between air traffic controllers.
The differentiation between various scenarios is done by voice communication, there is no need to complicate the squawk system. Typically there is not more than one aircraft squawking 7700 at the same time in an airspace so the identification based on code 7700 is simple and effective.

If you check on sites like http://avherald.com/ you will see how many incidents are worldwide - most of them are only incidents (and ending in a safe landing), but even then they will be considered an emergency while still in the air.

This is a comment, not an answer.

Isn't the purpose of an emergency call to be used only in really critical scenarios? How is an aircraft going down to be differentiated from one carrying a sick passenger?

The purpose of an emergency call is any situation where the aircraft needs special considerations for it or its occupants to remain safe.
If the plane is going down, the pilots will be struggling to regain control of the plane and won't even think about declaring emergency because nobody can help them with that anyway.
Where declaring emergency is useful is when ATC can actually help. Normally ATC tells pilots (under instrument rules, which all transport aircraft normally follow) where to fly. But when the pilots declare emergency, the controller gives them priority and now the pilot says what they need to do and the ATC will clear the way for them.
Depending on what the nature of the emergency is the pilots will request different things. If one engine quit, they'll probably request returning using turns towards the working engine only, if the gear got stuck they'll request low approach so someone from the tower can look at them with binoculars and tell them some more info about the state of that gear so they know better what to expect on landing, and then probably ask to do some circling to burn off fuel to reduce landing weight, and if a passenger is sick, they'll just ask for fastest possible approach and for ambulance to await them to take the sick to the nearest hospital.
But in all those cases, the point of declaring emergency is to request priority. So there is one squawk code to indicate that.

"If the plane is going down, the pilots will be struggling to regain control of the plane and won't even think about declaring emergency because nobody can help them with that anyway." This isn't always true. Many crashes are the results of things that the ATC can help with, or that don't result in that sort of immediate loss of control (e.g. running out of fuel, engine failures, etc.).

@nick012000, but in that case the plane is far from “going down“ at the point the emergency is declared. The situation can still go from bad to worse and the plane might eventually crash, but it didn't look like it's about to crash when the emergency was declared – which is another reason not to distinguish emergencies: they can go from bad to worse quite easily.


text:
What floatplane is this model?
What floatplane is this model?

(own work)

@JuanCarlosVelasco, you have several questions like this, where you show views of a 3D model you say is your own work. If it is something you have done you must know the answer already, in which case you are using this site to showcase your own work. If you truly do not know the model then this cannot be your own work. I think it would be good for you to clarify the basis of your question.

@GdD I assume he means the photography is his own work.

@qqjkztd he seems to have started adding it after being asked to cite the images.

@GdD I being asked to cite the images as my own work

@GdD I have a collection of approximately 1,200 airplane models, none of them have been assembled by me. I'm trying to identify the name of each of them. I have been able to identify the majority through books and aviation websites. Only when I can not identify an airplane I ask in the forum. All the photos that I upload are taken by me.


text:
What WWI biplane is this model?
What WWI biplane is this model?

(own work)

My first guess (but it is a guess) is an AirCo/de Haviland DH.4.
Reference: https://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=448

source
It looks to me like the British Martinsyde Elephant. 271 were built.

Avro 504. You can see a photograph of the original here.

Several avro photos seem to suggest the engine casing is almost circular and the tail having a funnier shape when compared to the photos above.

@Thunderstrike: That appears to be correct. https://www.youtube.com/watch?v=I9k7yxab6H8


text:
Why can't you operate packs on HIGH during take-off, approach and landing on a 737-800?
It's in the FCOM Limitations Chapter and I think that these phases are critical phases of flight and I don't want to reduce my engine performance by purposely creating a high demand for bleed air from my compressors.
Just want to seek some inputs on whether my chain of thought makes sense.

Yes that is about it.  Bleeds can impart a significant performance penalty because of energy losses from the bleed that limit thrust.  Not posting as an answer because I don't have 737-specific information.

alrighty thank you sir, it makes good sense and further more, if i leave my wing anti-ice on for takeoff, once it lifts off the ground, the air-ground switch sensor will shut the anti-ice.


text:
What is the late BF109's (G models) internal fuel capacity?
What is the late BF109's (G models) internal fuel capacity? I've been trying to google the answer for a few hours with no luck, perhaps someone here has the right source

The BF109G could carry 400 liters of fuel internally, and up to two 300 liter drop tanks externally.
You can also find the information in the flight manual, but you have to be able to read German. It's on page 7, under "Kraftstoffanlage".


text:
Is there an aircraft that can go supersonic at sea level without using afterburner?
Is there an aircraft that can go supersonic at sea level without using afterburner? Quick look through the "supercruise" capable aircraft might be helpful in narrowing down the possibilities, though I haven't seen anything specifying the top speed without afterburner and at sea level simultaneously. If I remember correctly, the F-15 is not capable of doing that; this gives you an idea, however, it's not the fastest jet in the world, so I'm still looking for the answer.

Simple answer...no, aircraft can only go supersonic when flying and if it’s at sea level it isn’t flying!

Aircraft yes, missiles.

@Notts90: There are places below sea level where flying is possible. The Dead Sea, for example.

@Notts90. “sea level” is a barometric (pressure) altitude, not an altitude referenced to a physical surface. It the altitude at which the ambient (static) pressure is 14.696 psia, which is the ISA altitude at 0 feet.  Altitude above ground level, AGL, is an altitude referenced to the local ground. On a day of low pressure, “sea level” will be physically higher than a day of high pressure. A flight level, FL360, is also a barometric altitude. Surely, you actually know that?

@Penguin yes quite aware, was a bit of a joke, deliberately interpreting it as “local” sea level, I do apologise.

@Notts90. No worries, but it did seem a bit like others “took the bait”!

@Penguin - No, actually "sea level" means the level of the sea.  For example, "MSL" altitudes do not change as barometric pressure changes.

The F-22 may have a shot at it.  It is also rated at 1.2 Mach down there.

According to Wikipedia, the BAC TSR-2 was capable of supercruise (no reheat) at Mach 1.1 at 200 feet (~60 m) altitude.  The TSR-2 first flew in 1964.  The English Electric Lightning had demonstrated supercruise several months earlier, but only at higher altitudes (above 20,000 feet, appr. 6 km).

Not sure if more than 1 ever flew, but versions of its Olympus engines were later seen in the Concorde.

The F-111 Aardvark could go Mach 1.2 at standard sea level conditions (14.7 psi and 59 deg F).  I worked on the F-111 as a design engineer at General Dynamics / Fort Worth TX Division (where the F-111 was being built) in the late 1960s.  At the time, the F-111 was the only aircraft in the world that could go supersonic "on the deck".  Of course, it required full military power and full afterburner, so it emptied the gas tank pretty quickly.

"Full military power" and "full afterburner" are mutually exclusive.

I would very much like to welcome you on this site, and won”t vote for deleting this answer.

Yes, indeed.  The F-111 had a thrust to weight ratio of greater than 0.6, as compared to 0.4 of the F-86. This answer gives a good idea of how much "push" it took at sea level.

He did specify no AB.


text:
What can cause a watchdog failure in an INDRA MSSR?
I am responsible for Mono-pulse Secondary Surveillance Radar (MSSR) system that resets itself after a watchdog failure.
So far I have found the following:

An unwanted number of replies are received by it.
A noise test was also done but the test didn't find any interference,
The system is running smoothly, but suddenly from west direction an excess number of replies are received which cause my MSSR to reset.
this interference time is only occasionally, it come out whenever they wanted,
The Frequency Allocation Board team has been contacted to find out the source of this interference,  but since it happens only occasionally they are also unable to find it.

If someone has experience on it or face the same issue kindly reply as soon as possible.

@Asifa Liaqat Welcome to aviation.stackexchange.com. I've edited your question so it is better readable. I hope my interpretation is correct, please make further modifications if needed.

<<this interference time is only occasionally, it come out whenever they wanted>> who's "they"?

1) What I expect is that there is a transmitter (or multiple) on 1090 MHz to the west of the RADAR that does not conform to the ICAO standard. I would start by looking at the decoded replies that the MSSR is receiving (not in ASTERIX format, but raw datagrams format. I believe the INDRA MSSRs can provide this). This may give a hint as to what the cause is(e.g. a certain 24 bit address range, certain downlink formats only, received power level, etc.).
2) Are there any military bases to the west of your RADAR? Or RADAR manufacturers? A transponder manufacturer? It may help to contact them. (Likely they will deny any experiments are going on, but the problem may suddenly disappear. It happened before).
3) Atmospheric tunneling may cause a specific transmission path only to exist under certain weather conditions (e.g. inversion layers) . Can you find any correlation between the interference and weather conditions?
4) Do you have multilateration systems covering the same area. The multilateration system may be able to locate the source.
If nothing else helps, you may have to install an array of 1090 MHz receivers and multilaterate the source.

MLAT system is installed and when interference came over MSSR, MLAT also received the same interference and yes at west side there is military base but sometimes this interference also received from east side

I have had a similar problem with a Thales RSM970S where a (hidden) QRSLS alarm caused a watchdog failure. The best we could conclude was that the interference was caused by non-compliant wireless equipment, but tracking the user proved to be impossible. With the help of the OEM we modified the bite alarm reference file to reduce the systems sensitivity to this issue (outside of the scope of normal optimization or LRU maintenance). The modifications had no negative impact on the radar's aircraft tracking, confirmed with SASS-C analysis.

I've encountered wireless security video systems from China that use 1090 MHz. They cause serious interference with secondary radar. You can imagine the joy when a local restaurant owner near an airport secured his restaurant with such a system... There are also video transmission systems used on drones (FPV) that operate in the aviation L-Band (e.g. Mateksys 1G3), close to the SSR frequencies and some PSR frequencies (although the PSR doesn't care much).


text:
What instruments or avionics do balloons usually have?
For normal sightseeing flights in a typical hot air balloon, what instruments or avionics does the pilot usually carry? Do balloons have a minimum equipment list?

The magic regulation for minimum equipment on a balloon (in the USA) is 14CFR31.85 ("Required Basic Equipment" for manned free-air balloons).
The FAA requires that balloons be equipped with:
For all balloons

[Reserved]
An altimeter.
A rate of climb indicator.

For hot air balloons (additional equipment):

A fuel quantity gauge.
An envelope temperature indicator.

For captive gas balloons (additional equipment):

A compass.

Beyond what's required by regulation, in most free balloon operations I've seen they also carry an aviation radio and/or a CB radio (to communicate with their ground crew) - this is usually a handheld transceiver.
The FAA Balloon Flying Handbook also makes for interesting reading. They talk about required and optional instruments and equipment found on various types of balloons in Chapter 2.

Additionally, to fly in controlled airspace: an aviation radio and a transponder. (BMA in Stockholm Sweden)


text:
How quickly can a hot air balloon climb?
What's a typical rate of climb for a hot air balloon, assuming stable air with no updrafts? I'm thinking of the classic 'sightseeing ride' type of balloon here.

The good balloonist will try to limit the vertical speed to 2 - 3 m/s (400-600 ft/minute) to make it easier for the passengers to accommodate to the pressure change and avoid ear barotrauma. This is the main factor; technically the balloon is capable of higher vertical speeds.
Much depends on the temperature profile of the atmosphere over altitude. The gas in the balloon will expand adiabatically, and the density difference to the surrounding air will change if the surrounding air doesn't cool at the same rate. For dry air, this cooling rate is 6.5°C per 1000m. If the atmosphere has a higher cooling rate, the balloon will accelerate, if the rate is lower, climb speed will bleed off the higher the balloon climbs.
For that reason, you will rarely see balloons flying at the same time as gliders. Gliders need thermals, which are best in air with a high cooling rate, whereas thermals will make the job of limiting vertical speeds in a balloon much harder.
Blimps need to keep their hull pressurized, and here the descent rate is limited by the ability to pump air into the ballonets.

I still wonder what is the theoretical maximum. Balloons in Kerbal Space Program can reach even 2 Mach vertically, which I seriously doubt is anywhere near to possible; I wonder what the realistic ballpark max is, though.

It very much depends on the balloon, but 2 metres/second in actual flight is pretty typical, while most are capable of something like 3-5 m/s ascent/descent if required. Ascent and descent speeds are normally about the same for a particular balloon, but this isn't a hard and fast rule
"sport" balloons being are capable of moderately less comfortable ascent/descent speeds in the range of 5 to 10 m/s


text:
If you were to fly an ILS in a knife edge would you receve GS and LOC be flipped?
because LOC and GS work the same way, if you were to fly one at 90 degrees bank would your information be displayed on the wrong indicators?

Don't fly the ILS all the way to touchdown like that though.

@Sanchises This pilot found a need to do just that. (See this also)

@CatchAsCatchCan, that video appears to be of an RC model, not a full scale aircraft. I am pretty sure it was already discussed somewhere around here.

@JanHudec I'm aware of that, hence my second link.

@CatchAsCatchCan: Here's a "real" (as in "not staged", but still an RC aircraft) example of that same stunt: https://youtu.be/uWoVKs5PljU .

Just curious... do you get reverse sensing if you are flying the ILS upside down?  Just a joke.

No, the directional part of ILS is located at the ground. (Ground) transmitter creates lobes with specified frequency and modulation to mark the high, low, right and left areas.
LOC and GS signals are distinguished by the carrier frequency, deviation from the optimal approach by relative intensity of 90 Hz and 150 Hz modulation in the signal. These signals provides information about position relative to the airport regardless of airplane attitude.
So flying knife-edge LOC axis of the instrument still shows (correct) horizontal deviation and G/S axis correct vertical deviation.
Of course, display of the instrument is turned 90° together with the whole airplane, so its G/S axis now lines up with the green-blue boundary you see behind your windows (I hope you are not flying such maneuvers in IFR conditions :) ) but it is still the G/S axis and it shows vertical deviation from the optimal glideslope.
Only difference would be that airplane antenna won't match the transmitter polarization (ILS transmitters transmit in horizontal polarization and receiver is likely polarized in the same plane), so you loose some signal intensity and receiver can be more sensitive to reflected signals, therefore reducing maximal working range and possibly increasing probability of spurious signal detection.

Well, the direction of the movement of the needles also won't match the direction in which correction shall be made, which is how I understand ”wrong” in the question.

@JanHudec would your information be displayed on the wrong indicators? GS would still show on the GS indicator and LOC would still show on the LOC indicator.

@TomMcW, but horizontal deviation would be displayed by the indicator moving vertically and vice versa.

@JanHudec Yes, but on the correct indicator. That's the way it's worded. The pilot would have to keep in mind the direction of the indication, but if he looked at the wrong instrument for guidance in each plane he could make the wrong directions.

@TomMcW, that's the problem with the definition of “wrong”. For somebody who understands how ILS works it is obvious what it should mean. But for somebody who does not it needs to be stated explicitly.

@JanHudec And here we are with terminology mess when flying unusual attitudes. :-) I have attempted to use explicit terms like "G/S axis" etc in my answer to make it clear, but maybe it does not turned out so well? OK, adding an extra note about the fact the instrument does not line up with outside references as usual. Not sure if it helps or makes everything only even more chaotic though.

As long as the aircraft accurately flies down the ILS, it doesn’t matter if it is knife edge, inverted, etc. The indications on the GS and LOC will stay centered. If you deviate from the GS or LOC the deviations will be shown on the respective instruments. The instruments don’t know or don’t care what the aircraft attitude is. The instruments only care about the deviation.

LOC and GS work the same way

They work on the same principle, but they each have their own radio signal. The localizer is on the frequency tuned in the radio (in range 108.10 MHz to 111.95 MHz), the glide-slope is on an associated frequency from the 329.15 MHz to 335.00 MHz (the mapping is not sequential for some reason). So the localizer needle will still show the deviation from the localizer signal and the glideslope needle will show the glideslope signal. They just won't be aligned with the directions to steer any more.
Neither also knows anything about direction. Each carrier has two tones broadcast on it (90 Hz and 150 Hz) that indicate left and right on the localizer frequency and high and low on the glideslope frequency. So when you are to the left of centreline, the receiver will sense more 90 Hz on the localizer and the localizer needle will move to the right relative to dashboard, no matter how your plane is oriented.
Also due to the nature of phase-shift directional antennas, there are other maxima around besides the main ones just around the intended flight path. One especially has to be careful about the false glide-slopes at 6° and 9°. The 6° has reverse sensing, that is above it the needle will direct you to the 9° one, below to the correct 3° one. And 9° is way too steep for any aircraft. There are similar maxima for the localizer, called side-lobes, but they are a bit easier to avoid by comparing your heading with the runway number.

I wonder if, because of the shape of the transmitted beams and the polarity of the signals and receiving antennas, if at some unusually high roll angle, the ILS receiver will throw flags because of a loss of signal.

To try to make things totally clear:
Suppose that the aircraft is making an approach in knife-edge flight, banked 90 degrees to the right. Suppose that the runway heading is north (it's runway 36).
If the aircraft is too low, the glideslope needle in the course deviation indicator will deflect "up" (towards the ceiling), indicating that the aircraft is too low. The pilot will correct for this by adding more left rudder input (which is to say, pressing harder on the left rudder pedal).
If the aircraft is too far to the east (right), the localizer needle in the CDI will deflect "left" (towards the left wing), indicating that the aircraft is too far to the right. The pilot will correct for this by giving down elevator input (which is to say, pushing forward on the stick).
The readings on the CDI will be correct in that they will correctly describe the location of the aircraft relative to the glidepath. However, they will not directly indicate what the pilot has to do; the pilot will have to recognize that, since they are flying in an unusual attitude, they will have to react to the CDI indications in an unusual way.


text:
What is the difference between Boeing 777-31H and Boeing 777-31H(ER)?
What exactly is the difference between a B777-31H and B777-31H(ER)? Is ER the latest version of 777-31H?

ER stands for extended range, which means added tanks and reinforcing the rest of the craft to hold all the fuel

and potentially weight savings elsewhere

The 777-31H is a standard Boeing 777-300 manufactured for Emirates (customer code 1H).
The 777-31H(ER) is a Boeing 777-300ER (Extended Range) again for Emirates.
Compared to the standard 777-300, the 777-300ER has:

A longer range due to the extra fuel it can carry (145538 kg compared to 135880 kg for the -300 baseline)
A higher Operating Empty Weight (167829 kg compared to 159570 kg for the -300 baseline)
A higher Maximum Take-Off Weight (351535 kg compared to 299370 kg for the -300 baseline)
A higher maximum structural payload weight (69853 kg compared to 64960 kg for the -300 baseline)
A wider wingspan (64.80 compared to 60.93 m)
A different engine delivering more thrust

The Boeing 777-300ER is always equipped with General Electric GE90-115BL1 engines, while the baseline -300 can be ordered with four different engine models from General Electric, Pratt& Whitney or Rolls Royce. The Emirates Boeing 777-300 aircraft have the Rolls Royce RR Trent 892 engines installed. They deliver 415 kN of thrust, almost 100 kN less than the GE90-115BL1 engines.

B777-300er has raked wingtip model, B777-300 has classic B777 wingtip model:

I suggest you can probably add some more description. As it is currently written, your answer does not completely answer the question. Please [edit] your post to add some more details.

B777-31H(ER) can fly further than B777-31H.

welcome to Aviation.se. While your answer is correct, it lacks detail, could you please edit your post to add more information?


text:
What causes a Cessna 172 and its gear to shake violently after landing and braking?
I am sure this was due to my own ignorance on proper technique, but immediately after landing in a Cessna 172, I applied brakes and some moderate aerodynamic braking and experienced an awful and violent rapid shake of the plane. I let off the brakes but the plane continued to shake for a few seconds.
This caught me off guard and certainly had my anxiety pretty high given I've never experienced this before. I cannot recall at the moment what I did to get the shaking to stop. I believe I simply let off the brakes and tried to steer the plane with the rudder. Again, this took a while (which felt like an eternity and had me wondering if I was going to lose control of the aircraft).
Once the shaking stopped, I tried to just lightly touch the brakes. The shaking started immediately again.
At this point I was very conscious about making sure I was using the most aerodynamic braking as possible and simply trying to steer the plane down the runway with no brakes.
What causes this? Are all Cessna 172s like this? I was advised when talking with a CFI to essentially not use the brakes until I've slowed down significantly.
For background, I'm used to flying the 4 seater Pipers (140, 160, 180, 200) and am transitioning to fly the 172's because my rental location no longer owns Pipers. I have never had this problem with a Piper. I could land, apply maximum aerodynamic braking and a good amount of toe brakes without issue (and optionally continually increasing the braking as the plane slowed more).
If I were to guess, I imagine this is due to the Pipers landing gear being short and stubby whereas the Cessna's gear is much longer and thus probably has more bend in the gear as well as torque it can impart on the aircraft.
UPDATE:
The nose wheel was on the ground. Sounds like a lot of you are thinking its nose wheel shimmy. I'm inclined to think you're right, but I'm surprised at the intensity of it because it was LOUD and things in the plane were shaking pretty significantly.

Paved, gravel, or grass strip?

This also happens to the 172's at the place I rent from, and the instructors just say "it's fine, don't worry about it", so I think it's just something that just happens. I don't know why, though.

Nose wheel on or off the ground? If it's off the ground, you should avoid braking as the reduced weight on wheels makes it easy to lock them up and flat spot them or cause wheel hop which could be your vibration. Normal procedure is to brake after the nose wheel is down.  If the nose wheel is on the ground, see @Jpe61 answer.

maybe it needs a front-end alignment? :)

@ZeissIkon, paved and well maintained.

How is the nosewheel on the ground during "aerodynamic braking"?

I'm pretty sure you have encountered "nose wheel shimmy".
For reference, see this video on Youtube: c172 M Nose wheel shimmy [sic].
The shimmy can occur for a variety of reason, most likely due to some imbalance in the tire/wheel combo, loose bearings and just the right caster angle induced by your braking (and thus pushing the nose down).
While it may not be a definite sign of an imminent catastrophic failure, the root cause should be investigated and dealt with, as it will get worse over time, and eventally when bad enough, quickly wear down nose gear parts causing it to fail.
Not to use brakes is not a fix to this problem, it's just someone being lazy and cheap, if I may say so.

Excellent choice of video! Shows how shimmy depends on wheel load and even gives advice on how to repair the issue in the comment below. +1

This shaking is the infamous Cessna nose-wheel shimmy.
It is a result of the bungee connection between the nose wheel and the rudder pedals, which is required due to the way you work the rudder of a Cessna in a side slip landing*. There is a shimmy damper on the nose gear, but they often aren’t maintained as well as they should be, and even in perfect shape they will only reduce it rather than eliminate it completely.
The proper way to handle it is to reduce the weight on the nose wheel, which also improves aerodynamic braking and puts more weight on the main gear, and gradually increase pressure on the brakes as the the plane slows down. Braking shifts weight to the nose gear, so you increase back pressure on the elevator even more to compensate.
For a short field landing, you are forced to accept some shimmy to get the shortest possible (book) rollout distance. You can also raise the flaps to reduce lift and add weight to the mains to increase braking action and add even more back pressure, but I wouldn’t add that distraction until you’ve got the rest of the technique down.
(* Do not crab a crosswind landing in a Cessna like you would in a Piper; learn how to do a proper side slip landing. The pattern work for that will give you plenty of chances to learn how to brake properly too.)

Yup, exactly what I thought, too. +1

Re "(* Do not crab a crosswind landing in a Cessna like you would in a Piper; learn how to do a proper side slip landing.)" -- care to say why a Cessna and a Piper should be any different in this regard?  Why is the Piper more suitable to be landed in crab than the Cessna?  Width between main gear, or something else?  Likewise why (if it's true) is the Cessna more suitable to be landed in a wing-down slip than the Piper?  Not enough wingtip clearance w/ the Piper?  Yet with the wider gear stance, I doubt that's actually true.   (This may end up going to chat, which is fine.)

As a person frequently accused by mods of gratutious over-editing, I have to say-- I really don't understand what was gained by the recent edit (not by the original poster of the answer), except for "bumping" the topic up to the top of the stack for no apparent reason after 2 years of inactivity.  (Maybe the ASE culture has different standards for edits to *someone else's" post?  Kind of makes sense, but... ?? )

I worked at an airport with 172 trainers for three years and got my PPL in those planes - they would shake all the time. If you didn't pull back on the yoke after landing, it was pretty common. Once they got to the point that you couldn't stop it from happening on landing or it got rather violent, they would go in and get a new damper on the front wheel.
All of this is to say it's not uncommon, but if it's bad enough to be concerning, go get it checked out. While it's likely a bad damper, it could be something more serious.
Here is a good article from AOPA on this issue: https://www.aopa.org/news-and-media/all-news/2016/september/flight-training-magazine/how-it-works
The important part:

The first time you experience a failed shimmy damper on the taxiway or
runway, you’ll think the aircraft is shaking apart. The important
thing to remember is that while it isn’t, and you’re safe, you need to
get weight off the nosewheel by pulling back slightly on the control
yoke. Right now.

Welcome to aviation.SE! I edited your answer slightly; please use the quoting markup for quotes, not the preformatted (code) markup. That helps screen readers and other tools work correctly.

Welcome!  You're doing great work, providing relevant personal experience that directly relates to and answers the question.  Keep it up !

In the case of disc brakes, braking shimmy can be caused by a brake disc that is warped or unevenly worn. I don't know if Cessna nosewheels have disc or drum brakes, but this is the most common reason for brake shimmy in ground vehicles, like my old Suzuki GS1000 motorcycle.
By the way, the technical name for nosewheel shimmy due to elasticity of the tire or the steering mechanism is caster instability.

No brake at all on the C172 nose wheel.

@StephenS, good, then it is not brake shimmy but caster instability.


text:
What percentage of airplanes are involved in a crash in their lifetime?
The chances of a commercial flight crash are about 1 in 1,200,000 - pretty low. If each plane makes about 40,000 flights in it's lifetime, about 1 in 30 airplanes will be involved in a crash before it reaches retirement.
Is this an accurate estimate? 1 in 30 seems like quite a large amount of airplanes to crash before they are retired.
I'm also assuming here that after a crash an airplane is written off. This may not be the case - I don't know how much a plane can be repaired.

1 in 1,200,000 where did you get that number?

@Simon https://curiosity.com/playlists/how-do-people-survive-plane-crashes-o53cN3Xy/#intro-playlist click "show more" - or use Ctrl + F for 1.2 Their citation is now broken.

@mins yes that answers one part - how many crash. That doesn't answer this question - what percentage of airplanes crash before they finish service.

Great question! So, statistically, flying is very safe if you are a passenger, but things look different when you are a plane (or a pilot!)

I just ran some numbers based on Wikipedia statistics. I don't know how they define "incident or accident" but hull loss is pretty straightforward. That means it was written off.

Aircraft type
Total made
Incidents/accidents
Hull losses

DC-10
368
55 (14.2%)
32 (8.3%)

L-1011
250
32 (12.8%)
11 (4.4%)

727
1832
336 (18.3%)
118 (6.4%)

737
8725
368 (4.3%)
184 (2.1%)

747
1514
131 (8.6%)
60 (4%)

All of these were introduced in the late 1960's. As you can see, the DC-10, L-1011, and 727, which are mostly retired, have pretty bad numbers. But the aircraft that are still being made and flown have much better stats. I think this is probably due to the fact that air travel gets safer all the time. The planes that had their heyday in the 70's and 80's fared poorly but the longer they continue to be produced, the less likely they are to crash.

Probably the best aircraft for measuring lifetime hull-loss rate is the 737 Classic: out of production long enough for most to have retired, but recent enough for much of modern air-safety practice.  It's suffered 50 losses out of 1,988 made, for a 2.5% loss rate.

The list is missing Airbus a/c. I'm reading a document from Boeing (which link was provided by @Peter Kämpf in this question). Page 14, for the period 1959-2014: 1890 accidents, 616 fatal, 958 hull losses. For 2005-2014: 404 accidents, 72 fatal, 165 hull losses. This is for "jet aircraft".

@mins Yeah, I didn't think of the A300. It's been around for a long time too. Your link to the document actually links to the question. Are you talking about the document linked by Mark in that question

@TomMcW The Boeing document link. Thanks for the feedback. In my previous comment I forgot to include the conclusion... hull loss seems not synonymous for fatality (which I expect to be more or less equivalent to crash).

@mins That chart is great. Problem is it's broken down by departures and the OP asked about per plane. I noticed that the hull loss numbers are way different than the Wikipedia numbers I used. Wiki says 118 hull losses for 727; Boeing claims only 94. Wiki says 32 for DC-10; Boeing shows 27. 11 vs 4 for L-1011. That's a pretty big discrepancy. I can see where "accident" can have different definitions, but hull loss is hull loss.

Do remember that the majoirty of hull losses are not crashes.

2013 worldwide fleet size according to Boeing: About 21.000 a/c (source). Another study: 23.000 in 2014.

@TomMcW: Does "Hull loss" imply a crash while piloted, or would it also include calamities that occur while the plane is parked?

@supercat In his comment Simon says that the majority of hull losses are not crashes, which is surprising. But I was looking up the L-1011 hull losses to see why the numbers between wiki and Boeing didn't match. Two of the hull losses were due to fires in unoccupied planes, three had bad damage in flight but managed to land, one exploded while they were scrapping it, one was downed by a bomb and 4 were legit crashes. So no, it does not imply a crash. On a personal note, I saw one of those crashed L-1011's still sitting beside the runway in ashes at JFK when my flight landed.

In case nobody can find official figures, let's try to make a rough estimate:

From the data in the IATA study cited in the question What are the statistical probabilities of commercial aircraft accidents?, we know for year 2014 there was:

100.000 flights per day,
A total of 12 accidents with casualties,

According to this study, there were about 23.000 aircraft in service in 2014,
Let's assume a mean 30 years aircraft lifespan for the aircraft in service in 2014,
Mean flights per day per aircraft = 4.3,
360 fatal accidents during a period equivalent to an aircraft lifespan.

That's a rate of 1.56 % aircraft with a fatal accident, or 1 / 64.
During this time:

The aircraft involved in the accident will have flown 47,000 times,
The 64 aircraft will have totaled 3 millions flights.

Said otherwise: an significant airline with 320 (5 × 64) aircraft:

Will face a disaster every 6 years,
After 3 millions flights (based on 2014 statistics).

(updated with a worldwide fleet of 23.000 aircrafts instead of 44.000 initially estimated.)

Again, seems quite a high chance so I guess it is significantly overestimated. I'd be interested in an official figure if it exists, but +1.

That's not so much. A company with 122 aircraft will face a crash each 30 years. Air France with their 350 a/c will face 3 crashes every 30 years. This seems to be the correct order of magnitude.

Yeah I suppose. Better than 1/30.

@Tim: I've changed the number of aircraft, and now the result is more 1/64, closer to your figure. But that's on a base of 4.3 flights per day, which seems quite high.

@mins I wouldn't say 4.3 is high. Doesn't work for international widebodies but for short haul and regionals the aircraft do more legs per day. I've done plenty of 4-leg days with another crew taking the airplane after us to do more flying.

The thing about statistical chances is that they reset every time the event did not happen. If a passenger has a chance of one in a billion flight hours to die in a crash, it does not mean that his chance of dying increases every time he flies.
In the casino, it is useless to track how many times red came out at the roulette - eight out of ten last balls came out red so now black has a higher chance, is not valid. Every time there is a 50/50 chance, history does not matter.
So if the plane has a chance of 1 in 1,200,000 to crash, it has that every time it takes off. It's not counting down the take-offs until it reaches 1,200,000 and then crash. It's number of crashes divided by a denominator like number of flights, number of flight hours, number of male flight attendants on board...be very very careful with denominators, and with projecting their meaningfulness.

On the other side, if your plane crash on the first time you fly, to you it's 100%.

Yes, game over.

"There are lies, damn lies, and statistics" -Mark Twain. Your point is very valid.

"If your plane crash on the first time you fly, to you it's 100%". That reminds me of those nuclear safety experts, saying: This sort of tsunami happens only once a century, so we don't need to worry too much for a plant which is supposed to last only 30 years.


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For the purposes of complying with an AD, what exactly does "missing and/or incomplete maintenance records" mean?
I'm considering the purchase of a Piper PA-28R Arrow. I calculated the factored service hours which came up to around 2500 or half of the required FSH to make it subject to the recent Piper wing spar AD 2020-26-16.  The AD does also require aircraft with "missing or incomplete" logs to comply by performing the inspection outlined in the AD.
The aircraft in question has logs going back to the year of manufacture, however, there is one small gap in the late 70s. A subsequent log entry states previous log lost in hangar fire. The gap is about 6 months in duration, and there were approximately 70 hours accumulated in that time.
Does this small gap in the logbooks make this aircraft subject to the inspection requirements in the AD?  I would think that since only 69 hours were accumulated that it would not, but I'd appreciate any expert opinions on the subject.  Thanks!

Seems like the only expert opinion that matters is the local FSDO's opinion. Not to mention that, even expert opinions are still opinions, which is off-topic for a Stack Exchange site. You should modify your question so that it can be answered factually with citations.

What does the mechanic doing your pre-buy say?  You are getting a pre-buy inspection done, right?

Good thought on contacting the FSDO.  I have someone lined up for a pre-buy, but haven't contacted him for details on what he includes yet.  I'll add this to the discussion.  Thanks for the suggestions!

Inspection of logs and AD compliance is normally part of the work your mechanic does on the pre-purchase.   You should ask the mechanic about it now to see if that is any kind of red flag from the get-go that could induce you to walk away, before they get started in the inspection and you have money committed.


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How does a commercial helicopter pilot get a Light Sport Aircraft type rating?
I hold Commercial Pilot certification with over 4,000 helicopter pilot in command flight time. What would be my flight training requirements to obtain a type rating for Light Sport Aircraft?

Welcome to aviation.SE! I think your question is essentially the same as this one; does that help?

I've added the FAA tag because it seems very likely you're asking about FAA requirements; for questions about regulations etc. please always tell us which country or regulator you're asking about. If I got it wrong, you can simply roll back my changes or edit again yourself.

If you're looking for going to be flying a Rotorcraft category LSA (gyroplane) and you already have gyroplane class privileges, then you already qualify. If you are looking to operate Airplane category SEL class, you need to add those privileges, either with an Airplane PPL or Airplane Sport Pilot aeronautical experience and checkride.

If he is a rotorcraft pilot but not ASEL he will need more than a checkride.

That is a good point. I've edited my answer.

A new checkride (at least with a DPE) is not required to add Sport privileges in a new category/class.  See 61.303 and 61.321.  You only need some endorsements from two different CFIs.


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What’s the most efficient way to earn the drone / 107 qualification for a retired ATP Pilot?
As an ATP-rated retired pilot (30 years experience) and a recreational drone pilot, I'd like to get the type for UAD (drones).
I am not 61.56 current.
What would be the most time- and cost-efficient ways to do this?

Welcome to Av.SE! Some may be put off by a question asking about "the best way," since opinion-based questions are a no-no here. I think, though, that a word-smith to "time-efficient ways" or "the most cost-efficient way" to do this makes the question sufficiently objective that it should be answerable with facts rather than opinions. And, as a guess, I suspect you aren't at all the only pilot with this sort of history & experience who'd be interested in getting such a qualification.

I took a stab at editing the question; if my edits missed the mark, you can roll them back or add further edits as you prefer.

@joeB, just for clarification, you do not believe that you would be able to complete a flight review and thus qualify as an existing Part 61 certificate holder?

To amplify @KennSebesta’s point, to get your Part 107 Certificate you don’t need to hold a current medical, just a pilot certificate and a current BFR. Then take the online course/test.
The other alternative is to take a written test on basic airspace rules (§107.73   Knowledge and training.) at a test center then take the online course/test. The written tests cost about $140 now and you must sign up for them through the FAA website.
Unless there is some reason you can’t get a BFR, I’d go that route because it’s more fun to talk about airplanes with a CFI than sit in a room and take a test, but it will probably cost more like $300 depending on your access to an airplane.


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How does a headwind affect the distance that a modern passenger jet can glide?
If in an emergency and a modern passenger aircraft had to glide, will it make a big difference if the aircraft had a headwind rather than having to glide downwind to nearest airport?

Any tailwind will increase your gliding distance. Any headwind will decrease your gliding distance. The stronger the wind, the more it will have an effect.

Welcome to aviation.SE! I suggest you have a look at this question and this one to see if they help.

@Pondlife the first link doesn't consider wind and the second is wrong in that it only considers best glide speed. The speed-to-fly is what we want to know for how far the plane can glide, and that is heavily influenced by head and tailwinds.

Maybe
The only way I can think to characterize a "big difference" for an airplane which has lost its engines is "Can the plane make it to the runway in time?"
If that's the question, then the answer has to do with the plane's glide polar.
Theory
The way to read the below chart is that the airspeed is on the x-axis and the sink rate is on the y-axis. You'll notice that as airspeed decreases, sink rate decreases; up until that sharp hook, which is when the plane starts stalling.
The best glide range occurs when the straight line is tangent to the L/D curve. (green curve in the below picture)

The straight line's starting point is determined by the atmospheric conditions:

Calm air: the starting point is the origin (red line)
Sinking air: the starting point moves up the y-axis (yellow line)
Rising air: the starting point moves down the y-axis (not shown)
Tailwind: the starting point moves back the x-axis (black line).
Headwind: the starting point moves forward on the x-axis (purple line)

Application
So the question of how much of a difference it makes comes down to how far the headwind vs tailwind moves the tangent line's starting point. You can see that the stronger the headwind, the steeper the tangent line. You can also see that a headwind has a very similar effect to sinking air, only the difference is that you never fly out of the sink.
As a results, a headwind has a surprisingly large impact on glide ratio. How much is dependent on how far the plane needs to fly and how much of a headwind it has vs. its glide polar.
Unfortunately, as is the way of these things, a headwind hurts much more than a tailwind helps. From my experience in gliders, I'd hazard a guess that a headwind which is only 10% of the best glide speed will have a >20% impact on range. 10% of an airliner's speed is quite fast, but it's not crazy fast and if it were flying in or near the jet-stream the plane could easily be fighting headwinds much stronger than that.
One last consideration in determining the glide range is how winds will change in both heading and direction as the plane descends. Check out the altitude slider at www.windy.com to get a feel for how the global winds vary with altitude.

The answers are amazing!

@Tevios glad it's helpful. Don't forget to upvote answers which are helpful, and to designate one as the accepted answer (if so appropriate). Although I'd give it some time yet, no need to rush. Others may yet be preparing their answers, too.


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Could we use ground effect (over water) to increase an airliner's range?
Theoretically, would it be possible to increase the range of a modern jetliner in ground effect over the sea?

It's important to consider that a jet engine is more fuel efficient at higher altitude.  An aircraft designed to fly in ground effect probably wouldn't be designed to fly at modern airliner speed though.  Commenting instead of answering because I'm not going to put a lot of rigor in.

Short answer is no, but the Soviets did build a craft designed to use ground effect.  Ekranoplan:  https://en.wikipedia.org/wiki/Lun-class_ekranoplan

Thankyou for your comment. I appreciate it. I asked the original question not to find out if there is any practical application because I suspect there is not. However, in the case of MH370, its worth exploring all avenues.

Yes, but.
Flying in ground effect clearly reduces the power required to stay airborne. Accidents like the one in this question are only possible because airplanes can take off at altitudes above their ceiling, but then fail to climb.
This means flying as slowly as practical. Then the gains from ground effect  will be greatest.
But a modern airliner is built to fly fast. Now let's see what needs to be changed in order to adapt it to the new regime:

No wing sweep
Thicker airfoils. Mach is no longer restricting optimum airfoil thickness.
Less aspect ratio.
Propellers instead of jets.
Much less installed power, so engines are not heavier than needed and run at optimum efficiency.
A comparatively larger horizontal tail

In other words: This will be a radically different airplane, and it will need much longer runways or even catapults to take off with its low installed power. Next, transatlantic flights will take three times longer at least, more than a full day, almost like in the days when Atlantic crossings were flown exclusively by airship. But it will need less energy, so once energy prices dominate fares, an Atlantic crossing by ground effect will cost less than by conventional airliner. However, since one flight takes so much longer, a much larger fleet of airliners is needed to transport the same number of people, so capital cost need to be taken into consideration, too.
For an inspiration how this airplane could look like, the past offers a good example: The Dornier Do-X. I once read (sorry, no source) that passengers on a steamer heard its typical engine sound and began to scan the sky for this airplane, only to discover it flying below their own height in ground effect, almost skimming the waves.

Do-X after liftoff (picture source)
If you now ask the passengers which mode of travel they prefer, only the very cheap ones will pick the ground effect airliner. The ones which contribute the major share of airline profits will prefer the faster route through the stratopause.

That plane is flying lower than most large bulk cargo ships sailing these days...


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Why can't blimps be economical for commuter / public transportation?
I dont know what the net load is for a blimp, but if they could somehow carry 100-400 people, why coudln't they be used like ferries in metro cities like SFO and NYC? Even if they could only scoot at 60-100mph, they can cut clean across waterways and save a ton of time for commuters while (seemingly) being more efficient than fixed-wing aircraft as well as safer (engine failure? gently float down and call for help).
A bonus would be if they could convert their balloon into a flying wing shape in flight via compressing the gas, reducing volume, and having it fold into a wing-shape allowing them to increase speed. If they could get near 200mph, that could open up decent commuter service between LAX <-> San Diego. But even 60mph could connect the greater Los Angeles area. If the weather were bad, a cable-tethering system could be used to guarantee smooth landings (fly into the tether, grab it, command the cable to retract, thus pulling the blimp into the station).

Note that, just because something may be economical in the long run, a large investment up-front may deter potential investors, especially if they can invest in existing profitable alternatives. So just because they don't exist doesn't mean they can't be economical.

Pondlife's link covers it pretty well.   A commercial product has to be able to fly whenever it's needed, whether scheduled or charter.  A blimp/airship transportation service that is grounded then the winds are above x knots (the x being not particularly high) would be like a bus service that can only run, say, when the roads are dry.  Just too unreliable as a transportation method.

I'm guessing that it would absolutely be possible to make blimps economical. The question is whether it would be better than alternatives.
For public transportation, there are a number of common alternatives:

Buses. Low-to-medium throughput, relatively low speed, short distance, very high flexibility.
Trams. High throughput, relatively low speed, short distance, high flexibility.
Metros. Very high throughput, medium speed, medium distance, medium flexibility.
Planes. Medium throughput, high speed, long distance, medium-to-low flexibility.
Ferries. Medium throughput, relatively low speed, situational (i.e. water connections).

(There's also regional coaches and trains, which are the big brothers of buses and metros. Omitted mostly for brevity.)
The challenge for blimps would be to out-compete any of these other forms of transportation. It would be impossible for a blimp to cover the very high throughput offered by metros or trains, or the flexibility of buses. For larger distances, planes are much faster and are not nearly as affected by weather. They also have the benefit of trained personnel/drivers (mostly) readily available, more infrastructure, production and we know that they work. Plus you don't have to change people's habits, which is hard.
In short, there is nothing a blimp could offer that isn't already covered by better alternatives. Except maybe a nice view on a sunny day.


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Does the retirement age for commercial airline pilots vary across the globe?
Do the retirement age of commercial airlines pilot varies from country to country?

Yes, they do vary between different countries. The international standard for regulations is created by ICAO, but member nations are free to deviate from ICAO rules and several do. ICAO currently limits retirement age to 60 for single-person crew, 65 for multi-person crew, and an exception to 70 for balloon or sailplane.
EASA did a study in 2019 in response to requests from countries to increase the age for single-person helicopter crews to 65. The link to the actual report is broken, but there is a summary posted on the ICAO website. The rationale from these countries is that standard retirement age in those countries is 65. The report lists a few large agencies that deviate from the ICAO standards. Japan raised their limit to 68 for multi-crew ops. Canada, Australia and New Zealand have no age limit, the latter two indicating that it would be considered age discrimination.


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Why do countries place limits on what foreign-registered aircraft can do?
Just read in local news Israeli regulator is giving a hard time on a company operating regularly out of Israel a plane with foreign (US) registration.
The company wants US registration to get better freedom to fly to various countries. This I understand. But what difference does it make otherwise? Why would the regulator care, and make it difficult to operate regularly without local registration? Are such limitations common in other countries?

In many ways this isn't an aviation question as much as a political and economic one. If you run a business in Israel, why can't you register your company car in France (or any other country you choose)? Most countries want to have legal and economic control over people, companies and goods based there. Without that, there would be no way to enforce local safety standards, collect taxes and other fees, and generally run the country. There may be some interesting parallels with shipping and flags of convenience, but law.SE or politics.SE might be better places to ask about that.

Perhaps the biggest reason for this is that countries have different standards of safety.
One example, is a few years ago, Cessna recommended that basically all their piston aircraft have a thorough maintenance inspection known as a SID once they reach 20 years of age. The FAA didn't feel that these inspections justified the fairly considerable cost. Other countries though - Australia, NZ, Germany come to mind - felt the manufacturer knows best, and made the inspections mandatory.
This is just one example of the broader problem of differing standards throughout the world. When an aircraft gains Israeli registration, it must comply with all local laws and airworthiness directives, giving authorities confidence that their standards are being met. Being locally registered also makes it easier to track movements and to target inspections. There could also be tax implications too, as it would become a local asset.


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Taxi to spot?? Hot spot? What is meant by this?
I heard ground controller at KPHL say taxi to spot so and so?
What do they mean by this during taxi procedures.

Can you provide an audio link or something? It's sometimes a little tough without context

The word “spot” just means a designated place, or part of the airport where the aircraft is to be parked and shut down.  Sometimes they are numbered.  Presuming the pilot knows where that is, the controller is simply clearing them to go there.

A spot will be just a place on the airport surface. Generally these are termed as specific locations such as the blank ramp for example. A “Hot Spot” is a specific location that is a known hazard for aircraft for one of numerous reasons. These reasons could be from a history of confusing pilots or just from common sense and logic. Some Hot Spots are designated where several taxiways and or runways intersect. They can be Considered hazardous because pilots in the past have confused taxiways for runways and/or runways for taxiways. Some Hot Spots are considered hazardous because Towers view of that area is blocked by obstacles or obstructions. Known Hot Spots are marked and numbered on some airport taxi diagrams to alert pilots of the hazard.

The airport layout is represented in terms of aprons, taxiways, Ramps, gates, bays and sometimes uncommon words like loops, spots etc. The same is very clearly marked on airport charts and is easily locatable if one cares to look.
Hot spots are typically intersecting taxiways where there is a possibility of conflict of aircraft on the move. These are also marked on the airport charts and are recommended to be briefed between pilots.


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How does 'standalone' propeller efficiency change with altitude?
I've seen questions regarding propeller aircraft and altitude and their interaction. But I'm wondering what the relationship between just the propeller and altitude (changing air density) is. How does the efficiency and ability of a propeller scale with altitude?
Real-life scenario which inspired this question: a two-bladed two-engine propeller system is being used to propel a payload on a balloon platform (i.e lift is handled by helium buoyancy, no minimum speed required to stay afloat). The propeller is only used to move horizontally slowly (< 5m/s). This occurs at 65,000 feet and the propellers were analyzed at these conditions. Now, the new altitude is going to be 85,000 feet. How does that affect the scaling and re-design of the propulsion system? The Reynolds number will of course be different for the same desired airspeed. Additionally, the system drag will be lower so less thrust will be required at the same airspeed. But if the required thrust needed was the same, how does that and other factors affect the efficiency? And knowing that the required thrust will be lower scaled to air density, what is the expected change in propeller requirements?
Addendum: can the same 65,000 feet propellers be used at 85,000 feet, or is a new design required?

Being limited to sub Mach#, you are already at a wall in terms of blade velocity, so I think it's just a matter of needing more and more blade surface area the higher you go. Somebody here probably knows how to do the calcs.

You may wish to consider the electric 2 hp 2 meter "laminar flow" Helios props, good to 95,000+ feet.  These propelled a wing to sustained flight at that altitude, and may be a good "off the shelf" solution.

Yes, the same propeller can be used.
Propeller thrust is proportional to air density. The same is true for aerodynamic drag, so your propeller will work at all altitudes equally well. Since the speed of sound drops with temperature, you need to pick your propeller tip speed for the coldest layer, which normally is the Tropopause. There, the speed of sound is 295 m/s and if the propeller tips don't exceed 85% of that, efficiency should be roughly constant over altitude. A small loss in efficiency with altitude must be accepted, however, since the propeller's friction drag increases with altitude.
This would not be true for airplane propellers where the drag of the airplane they are propelling is constant (actually, rises a bit) with altitude, but since your thrust demand drops in line with density, no new propeller is needed at 85,000 ft.

I don't quite understand the second paragraph. Why does the drag for airplane propeller stay constant (or slightly increase) with altitude? Do you mean that the aircraft drag drops by about slightly less of a factor as lift due to the altitude, and thus a slightly higher airspeed is required to counter weight, and that would require a slightly greater thrust?

@spaceprops What I meant is the drag of the plane the propeller is attached to. Since you don't need to generate lift, your thrust requirement over altitude is very different. I'll try to make the answer more readable.

wouldn't aircraft drag also decrease in proportion with density though according to the drag equation "0.5 * rho * V^2"

@spaceprops. No, because speed squared has to grow to compensate for lower density. The ratio of lift to drag is about constant (gets a bit worse with altitude) and lift must stay constant because mass doesn't change. Hence, the product of density and speed squared has to stay constant, too.

ah, right, thanks!


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Why can airliners not fly as fast as fighter jets?
Why do passenger aircraft not fly at the speed of fighter jets? What factors stop them from achieving such speed?

Speed means energy, means fuel consumption, means expensive, means no passenger :)

https://www.youtube.com/watch?v=n1QEj09Pe6k

There are 2 big reasons:

Speed costs lots of money. Fighter jets can go much faster than passenger jets but they spend most of their time cruising as the same speeds because it is more fuel efficient. Faster airplanes need more exotic materials and would be more expensive to build. The engines would need to be bigger and more powerful and would cost more, and to go very fast uses a lot more fuel as air resistance increases exponentially with speed. Concorde tickets were very expensive not because of any sort of exclusivity, but because that's what it cost to operate it. Newer technology could probably decrease that somewhat, but it would still be very expensive. Air forces can operate fast jets because they have the funding, airlines have to make a profit
Passenger jets fly pretty close to the speed of sound already, to go much faster they would need to break the sound barrier, which creates a very loud sonic boom that disturbs people on the ground and can break windows in some cases. Supersonic travel over land has largely been banned because of that, so unless you are over water you can't go any faster anyway

Passenger airplanes do fly at the cruising speeds of most fighter jets. Most fighter jets cruise subsonically between Mach 0.8-0.9 (to extend range), which is a typical cruising speed of jet transport category airplanes:

A320: long range cruise 0.78M, high speed cruise 0.8M
B777: cruise speed 0.84M
Global 7500: long range cruise 0.85M, high speed cruise 0.9M

Any faster, you'd reach supersonic speeds, which is prohibited over continental territories of most countries due to sonic boom noise concerns. This is aside from the much higher fuel costs of cruising supersonically, and an often prohibitive costs of developing a supercruise capable airliner.